The present invention relates to a circuit for controlling operation of a transmitter in a mobile communication terminal, which transmitter is designed for the modulation modes EDGE and GMSK, having:
a power amplifier which delivers an output signal for an antenna on the mobile communication terminal;
a phase controller, which has a phase comparator, to which a nominal signal for the output signal from the power amplifier is supplied, and a voltage controlled oscillator for aligning the phase of the output signal from the power amplifier, for controlling the phase of the output signal from the power amplifier;
an amplitude controller, which has an amplitude comparator, to which the nominal signal for the output signal from the power amplifier is supplied, and a battery voltage modulator, powered by a battery, for aligning the amplitude of the output signal from the power amplifier, for controlling the amplitude of the output signal from the power amplifier; and
a feedback line for feeding back a present measured value for the output signal from the power amplifier to the phase comparator and to the amplitude comparator, which feedback line has an adjustable amplifier, controlled by a baseband chip using a control signal, for setting an output power for the antenna. Such a circuit is used in the prior art both for the modulation mode EDGE and for the modulation mode GMSK, with the circuit being invariable with regard to such modulation modes. The way in which the known circuit works is explained below with reference to FIG. 2. It will be emphasized in this regard that a transmitter equipped with such a circuit is referred to, generally, as a “polar loop transmitter.”
As will be seen in FIG. 2, a nominal voltage signal Us is supplied both to an input on a phase comparator 1 and to an input on an amplitude comparator 2. Respective second inputs on the phase comparator 1 and on the amplitude comparator 2 have an output voltage Ua applied to them, specifically with the interposition of an adjustable amplifier 4 which determines an attenuation for the output voltage Ua and hence a gain factor Ua/Us for the amplitude control loop. The attenuation of the controllable amplifier 4 can, thus, be used to set the output power delivered to the antenna.
The adjustable amplifier 4 is controlled by a baseband chip in a mobile communication terminal to which the circuit belongs, with a control signal R applied to the adjustable amplifier 4 being able to be chosen on the basis of a power requirement from a remote station for the mobile communication terminal, such as a base station in a mobile radio network.
An output signal from the phase comparator 1 is supplied to a voltage controlled oscillator 5 whose output signal Uin is applied to an input on the power amplifier 3 and aligns the phase of the output signal Ua with the phase of the nominal voltage Us.
An output signal from the amplitude comparator 2, which is typically an integrating error amplifier in practice, is applied to an input on a battery voltage modulator 6 which is powered by a battery Ubatt or a storage battery in the mobile communication terminal. An output signal from the battery voltage modulator 6 is used for aligning the amplitude of the voltage signal Ua with the amplitude of the nominal signal Us, specifically taking into account the attenuation by the adjustable amplifier 4.
In the circuit shown in FIG. 2, a feedback line runs from the output of the power amplifier 3 for the output voltage Ua via the adjustable amplifier 4 to the respective inputs of the phase comparator 1 and the amplitude comparator 2.
The battery voltage modulator 6 directly influences a supply voltage Uc for the power amplifier 3 and, thus, an envelope for the output voltage signal Ua from the power amplifier 3.
The circuit explained above has no kind of variability for the purpose of optimizing this operation with respect to various modulation modes, such as EDGE and GMSK. These modulation modes are readily known to experts in the field of mobile radio technology. For details, reference is made to the specialist book “Mobilfunknetze und ihre Protokolle 1”[Mobile radio networks and their protocols 1] by B. Walke, third edition, published by Verlag B. G. Teubner, 2001.
The described circuit based on the prior art, which is used for linearizing the power amplifier 3, has the following drawback when used for either the EDGE or GMSK modulation modes: to ensure error-free operation of the transmitter, the power Pa delivered to an antenna 7 by the power amplifier 3, and thus the output voltage Ua, must always be below its maximum deliverable output power Pmax. If a drop in the available storage battery voltage causes, by way of example, the maximum deliverable output voltage Umax to fall below the value which is to be chosen for the output voltage Ua on the basis of the control, then the amplitude controller reaches its control limit. In this unwanted operating state, control loop instabilities and saturation effects can result in impairment of the switching and modulation spectrum. Timing problems also may arise, since when its control limit has been reached the amplitude comparator requires a certain time in order to leave saturated operation; for example, if the output power Pa needs to be brought down at the end of a transmission time slot. As such, both the appearance and the leaving of the control limit results in unwanted effects in the time profile for the output power Pa as well as in the switching spectrum, where even the prescribed power boundaries for relevant mobile radio standards can be infringed.
To avoid the control limit, appropriate dimensioning of the power amplifier 3 has been used, to date, to provide a sufficiently high reserve for the available output power Pmax. However, this results in high component costs and/or premature disconnection of the mobile communication terminal as a result of faster consumption of the storage battery capacity. The latter drawback can be overcome only by using a larger and more expensive storage battery.
Against this background, the present invention is directed toward a circuit, as cited at the outset, wherein it is economically more efficient for the two modulation modes EDGE and GMSK.